1. Field of the Invention
The present invention relates to an audio recording and playback device provided with a dedicated power feed function for an external microphone, and to a power feed method for the audio recording and playback device,
2. Description of the Related Art
Portable audio recording and playback devices for converting an audio signal to digital data, storing in a rewritable storage medium such as flash memory, reading out this stored audio data and converting to analogue for playback (hereafter referred to as IC recorders) have been in practical use since the latter half of the 1990's, and a wide variety of IC recorders have been marketed to date.
On the other hand, there has been dramatic advancement in the increased capacity of flash memory, and with recent IC recorders high sound quality PCM recording has become mainstream. Furthermore, professional level IC recorders provided with a female terminal for connecting to an external microphone connection terminal (usually called an XLR connector or Cannon connector), having excellent noise tolerance that is particularly significant on original sound recording, are being produced.
Many external microphones are provided with a balanced cable made up of three conductors, namely HOT, COLD and GND. Also, one end of a balanced cable is provided with male terminals of an external microphone connection terminal. Of the above described three conductors, two of them, namely HOT and COLD, also function to feed power to the external microphone, and a power feed method that superimposes direct current on the two HOT and COLD conductors is generally called “phantom power”. This phantom power conforms to JEITA RC-8162B (power supply methods for microphones) defined by the Japan Electronics and Information Technology Industries Association.
The female side where the above-described two conductors HOT and COLD are connected is provided with a high impedance differential input amplifier circuit (usually called an instrumentation amp). As a result, it becomes easy for large transient voltages in excess of the absolute maximum rating of the differential input amplifier circuit to occur at the time of starting or stopping power feed to the external microphone, due to sudden fluctuation in current flowing in the HOT and COLD conductors and in capacitors and resistors constituting the differential input amplifier.
Because this type of transient voltage has an adverse affect on devices, it has been proposed to control transient voltage. For example, with a signal amplifier disclosed in Japanese patent laid-open No. Hei 9-74333 (laid open Mar. 18, 1997), a passive circuit (CR integration circuit) is provided in an input circuit of a balanced microphone amplifier to which phantom power is connected, to control transient response voltage (also called click or pop noise).